This invention relates generally to lasers, and more particularly, to laser resonators of cylindrical or annular configuration. Obtaining high-power beams from conventional linear laser resonators poses almost insurmountable problems. The laser resonator must have a cavity length that is either impracticably large, or has to be made in a folded configuration that increases the number of mirrors required. Long cavity lengths can also result in degradation in beam quality, and such lasers are extremely sensitive to mirror alignment.
Another significant problem is that a conventional linear high-power chemical laser requires a linear flow of gases, which produces a corresponding linear accelerating force on the laser structure. In a well known form of a chemical laser, the principal chemical reaction is between fluorine and hydrogen, and produces excited hydrogen fluoride molecules and atomic hydrogen. Fluorine and hydrogen are typically injected at supersonic speeds, through nozzles into a resonant cavity, giving rise to the accelerating forces.
For these and other reasons, designers of high-power lasers have more recently shifted their attention to cylindrical or annular resonator configurations. Greater powers can be obtained more readily from cylindrical configurations, and the radial accelerating forces are distributed uniformly, and are therefore self-cancelling. However, the conventional linear laser with an unstable resonator has at least one important attribute. It provides inherently good mode control. Undesirable higher-order modes of operation of the laser are not present, and the laser therefore provides good beam quality. Cylindrical or annular lasers, although yielding higher powers, do not provide beams of intrinsicly good quality. If some of the power could be sacrificed, spatial filtering could be employed to remove unwanted higher-order modes of lasing, but spatial filtering is inefficient from a power standpoint.
Various designs and proposals have been advanced to seek, in effect, the annular analog of the conventional unstable linear laser resonator. The ideal annular laser resonator configuration would be one that combined the advantage of beam quality, which is inherent in the unstable linear resonator, with the advantages of high power and symmetry inherent in the annular configuration. However, as will be explained in more detail, annular laser configurations prior to this invention have been deficient in some important respects.
The common features of annular lasers are an annular gain region and an annular resonator. The principal requirement for the resonator is that it extract a large amount of power efficiently from the annular gain region, in such a manner that mode control, and therefore beam quality, are preserved.
The simplest annular resonator is the toric unstable resonator (TUR), which consists of two toric mirrors arranged at each end of the annular gain region. Since the toric optics have no single optic axis, there is no diffractive coupling in the azimuthal direction, and operation of the device is not satisfactory. Modifications to enhance mode control in the toric resonator have not been successful and the configuration has been largely discarded by investigators.
An annular resonator configuration known as the half-symmetric unstable resonator with internal axicon (HSURIA) was intended to provide the desired combination of advantages. It combines the principal features of the toric unstable resonator, but also includes an optical element known as an axicon to convert the annular beam to a compacted cylindrical one. One form of the axicon is known as a waxicon, named for its letter-W shape when viewed in cross-section. A waxicon is basically an arrangement of two approximately conical mirrors. A first, outer conical mirror with an internal reflective surface reflects the annular beam inwardly toward a second, inner conical mirror, concentric with the first and having an external reflective surface. A section taken through a waxicon shows the two conical mirrors in a letter-W configuration. The annular beam is reflected radially in toward the optical axis of the waxicon by the first conical mirror, and is then reflected in an axial direction by the second conical mirror, the effect being to compact the annular beam into a cylindrical one, directed back along the central axis of the original annular beam. The compacted beam impinges on a scraper mirror, which reflects a central portion of the beam back into the resonator optics, and allows an out-coupled portion of the compacted beam to pass. Instead of a waxicon, a reflaxicon may be used. A reflaxicon also has two concentric conical mirrors, but the inner one is in a reversed orientation as compared with the waxicon. In a sectional view of a reflaxicon, the two conical mirrors appear to be parallel, and the compacted cylindrical beam continues in the same direction as the original annular beam.
The basic HSURIA configuration includes a waxicon or reflaxicon element at one end of the annular gain region and a plane toric mirror at the other end of the gain region. The resonator cavity is formed by the waxicon or reflaxicon, the toric mirror, and the scraper mirror, and has the simplicity of its toric optics and a single optical axis in the so-called "compact leg," in which the cylindrical beam is propagated. However, the configuration also has some significant drawbacks.
Most importantly, the arrangement is extremely sensitive to the mirror alignment, and particularly to any degree of tilt in the toric mirror. Substitution of a corner cube mirror or a conic mirror for the toric mirror is sometimes made in an attempt to reduce this effect. In both cases, incident light in the annular beam is reflected from one side of the corner cube or conic reflector to the opposite side before being reflected back along the cavity. This poses a very serious polarization problem, in that the polarization of the light is scrambled by the conic or corner cube surface. A waxicon also inherently scrambles polarization, and it was ultimately discovered that the only practical modes of operation of the HSURIA configuration were either radially or tangentially polarized. As a result, the light beam out-coupled from the resonator tends to be self-cancelling at the optical axis. This, of course, is contrary to the normally desired far-field pattern of light generated by a high-power laser.
One solution to the polarization problem is to coat the toric elements of the resonator with special phase-shifting coatings, such that no net polarization shift is produced in a round-trip passage through the resonator. However, the use of coatings tends to aggravate manufacturing problems, since the optical elements have to be made to an extremely fine tolerance. In particular, the apex of the inner conical surface of the waxicon or reflaxicon may not be truncated without losing mode control of the device.
An alternative form of the basic HSURIA configuration described above is the traveling-wave or ring resonator version. Instead of a plane toric or conic rear mirror, another axicon is used to compact the annular beam and direct it to the scraper mirror.
Conventional annular optical resonators, typified by all forms of the HSURIA configuration, operate with less than a desired extraction efficiency, due to appreciable diffractive losses, as well as insufficient saturation of the gain medium. Moreover, the beam quality of the extracted radiation, as measured by the far-field radiation pattern, is not acceptable because of discrimination against higher order resonator modes. Accordingly, there is a need for an annular laser resonator configuration that addresses these problems. In particular, what is needed is an annular resonator providing improved laser power extraction efficiency with good far-field beam quality, and preferably no polarization problems. The present invention is directed to this end.